The present invention relates to an image forming apparatus, specifically relates to an image forming apparatus capable of forming a high quality image from beginning to restart to form an image.
There is known an image forming apparatus of an electronic photography recording system for exposing to form an electrostatic latent image on a surface of an image carrier executed by a photosensitive member, and according to the image forming apparatus, an image is formed by transferring a toner image onto a record medium such as a record sheet or the like. A developing roller opposed to the surface of the image carrier rotates to cause toner on an outer peripheral face thereof adhere to the surface of the image carrier to develop the electrostatic image as the toner image. The developing roller is supplied with the toner by bringing a supply roller rotated in a space that contains the toner into press contact with the developing roller to rotate.
The image forming apparatus adopting the electronic photography recording system is insertably and detachably mounted with a developing cartridge (developer) having a container containing the toner along with the developing roller and the supply roller, and there is also present an apparatus constituted to be able to contain a plurality of the developing cartridges at inside of a developing rotary unit. According to the image forming apparatus, the developing cartridge at a developing position at which the developing roller is made to be opposed to the image carrier can be switched by rotating the developing rotary unit centering a rotating shaft.
According to the image forming apparatus, as the developing cartridge for adhering the toner to the surface of the carrier, by constituting to enable to mount the developing cartridges respectively containing toners of yellow (Y), magenta (M), cyan (C), black (K) to the developing rotary unit, a color image overlapped with the respective colors can be formed by successively switching the developing cartridges (refer to, for example, JP-A-2004-126089). Further, according to the constitution, a monochrome image by a single color of an image, for example, a black and white monochrome image (hereinafter, also simply referred to as a monochrome image) by the toner of black (K) can naturally be formed. According to the constitution in which the plurality of developing cartridges contain the toner of the same color (mainly black), by successively switching the developing cartridges used by rotating the rotary unit at each timing of using up the toner or the like, the monochrome image can be realized to continue to form over a long period of time by reducing a number of times of operation of replenishing the toner by interchanging the developing cartridges (refer to, for example, JP-A-2002-351190 and JP-A-2003-316106).
However, according to the image forming apparatus, when a standby time period after finishing an image forming operation is long to some degree and the developing roller is left to stay in a state in which the developing roller is not rotated, on the outer peripheral face of the developing roller, a difference is brought about in a state of the toner on the outer peripheral face between a region thereof in which the toner is exposed to outside as being opposed to the surface of the image carrier and a region thereof in which the toner is disposed at inside of the container of the developing cartridge. Therefore, when a successive image forming operation is restarted, there is brought about nonuniformity in density in correspondence with the exposed region in which the toner is exposed to outside and the region at inside of the container, in other words, the nonuniformity in density coinciding with a rotating period of the developing roller, and there is a possibility of occurring a problem in an image quality.
In order to resolve the problem, according to JP-A-2004-126089, it is proposed to put the toner on the outer peripheral face of the developing roller to a uniform quality by counting a time period elapsed from an end time of the image forming operation at a preceding operation and idly rotating the developing roller to refresh when the elapsed time period becomes equal to or longer than a set time period.
According to the image forming apparatus described in JP-A-2004-126089, it has been found that although deterioration in the image quality by a longish standby time period can be realized to resolve, it is preferable to constitute a countermeasure therefor even when the standby time period is a short time period.
First, by charging the developing roller while repeatedly supplying the toner onto the outer peripheral face of the developing roller by the supply roller rotated at inside of the developing cartridge, the developing operation of adhering the toner to the electrostatic latent image on the surface of the image carrier is continued and therefore, the toner on the outer peripheral face is brought into a state of being saturated by a desired charge voltage.
The toner on the outer peripheral face of the developing roller is selectively adhered to the electrostatic latent image on the surface of the image carrier and therefore, the nonuniformity is occurred by whether the adhered portion is the portion in correspondence with the electrostatic latent image on the surface of the image carrier and a new toner is replenished to resolve the nonuniformity. At this occasion, the toner to be replenished is present at a surrounding of the supply roller and therefore, during a time period of repeating the developing operation, there is brought about a charged state which is more proximate to the toner on the outer peripheral face of the developing roller than the toner exposed at a position quite remote therefrom. Therefore, the electrostatic latent image on the surface of the image carrier can be developed by a uniform quality by bringing the developing roller into a uniform state by repeatedly replenishing a toner on the outer peripheral face.
However, in carrying out the final developing operation, when the operation is stopped in a state in which there is the nonuniformity of the toner on the outer peripheral face of the developing roller, there is brought about a difference between a state of the toner left on the outer peripheral face and a state of the toner replenished to resolve the nonuniformity on the outer peripheral face of the developing roller when the developing operation is restarted in forming a successive image. That is, it is conceived that whereas the toner on the outer peripheral face of the developing roller opposed to the surface of the image carrier is left in an electrically floated state, the toner at inside of the developing cartridge is brought into a state in which a large amount of the other toner is present at a surrounding and therefore, even when the toner is left in a short period of time, there is brought about a difference therebetween in a degree of charging. Under the state, even when the developing operation is restarted, the electrostatic latent image on the surface of the image carrier cannot be developed by the uniform quality owing to the toner on the outer peripheral face of the developing roller.
The nonuniformity of the toner tends to differ in characteristics in accordance with kinds of toners having different colors, among toners of yellow, magenta, cyan, black, particularly, the toner of black having a high frequency of continuously printing the monochrome single color image includes carbon and is excellent in conductivity and therefore, the toner of black is more liable to be the nonuniform than the toner of other colors and it seems that the more increased the content of carbon, the more significant the nonuniformity.
Owing to the difference in the conductive characteristic, the charge voltage of the toner of black differs from that of the toner of other colors and it is necessary to rotate the developing roller more than in the case of the toner of other colors until saturating the charge voltage. Therefrom, in the case of the toner of black, a number of rotations of the developing roller necessary for resolving the nonuniform of the toner tends to increase such that whereas the number may be one rotation in the case of the toner of other colors, in the case of the toner of black, it is preferable to rotate the developing roller more than two rotations.
Here, although as a plan for preventing the nonuniformity of the toner from occurring, it is conceivable to increase a charge efficiency (voltage) of the toner on the outer peripheral face of the developing roller, when the charge voltage is indiscreetly increased, developing performance is deteriorated in an image of the nonuniformity in density or the like.
Further, the possibility of bringing about the nonuniform of the toner is changed by the charge characteristic of the toner and therefore, the nonuniform is liable to be brought about by other factor of temperature or humidity of a surrounding environment, further, a degree of deteriorating the toner or the like.